Hypothesis: metronidazole gel applied topically after periodontal debridement in smokers volunteers could improve clinical parameters when compared to metronidazole tablets + periodontal debridement. Method: 30 smokers with chronic periodontitis were randomly assigned into 3 groups: periodontal debridement combined with 3 g placebo gel; periodontal debridement combined with daily topical application of 3 g metronidazole benzoate gel (15%); and periodontal debridement combined with a daily single dose of 750 mg metronidazole. Clinical parameters evaluated were visible plaque, gingival bleeding, probing pocket depth and relative attachment level.